1. Field of the Invention
This invention relates to methods and means for balancing rotatable units, and relates more particularly to a dynamically balancing and then retained rotatable unit, such as a disk file, in dynamically balanced condition.
2. Description of the Prior Art
The balance correction techniques traditionally used for rotatable units involve adding or removing weights or bending a tab or the like to adjust the center of mass.
The technique currently in widespread use to retain rotatable units, such as magnetic disk files, in dynamically balanced condition is to adhesively secure weights of selectable magnitude at different locations around the hub after the magnitude and angle of the weights needed for balance are calculated by a balancing instrument.
U.S. Pat. No. 4,075,909 discloses a plurality of balls movable within an annular raceway to adjust rotating shaft imbalance upon flexure of a radially movable floating inertia ring. When balance is achieved by repositioning of the balls, the inertia ring rotates concentrically with the shaft to trap each ball in circumferential position. This technique has the following disadvantages: (1) reduced sensitivity due to the need to overcome finger spring loads before the balls are free; (2) need for the balls to overcome friction to roll to a position of lesser imbalance; (3) reliance on precision of the inertia ring for minimum imbalance; (4) limited balance resolution dependent upon the circumferential spacing of slots in the periphery of the ring; (5) complexity of design and assembly; and finally (6) it requires use with a flexible shaft.
Another technique is disclosed in Russian patent 693,136. It uses an electromagnetic to generate wall oscillations so that ball weights can move freely in a damping liquid sealed in an annular cavity to compensate for rotor imbalance; whereupon the balls are locked in their positions by the electromagnet.
U.S. Pat. No. 4,060,009 describes a method of balancing a rotatable hollow shaft by providing within the shaft a sealed housing containing an annular cavity. In the cavity are a plurality of balls and a thermo-setting adhesive. When rotated above resonance, the balls reposition themselves within the cavity to counteract the out-of-balance condition; whereupon a heating coil or the like heats and sets the resin to retain the balls as repositioned. This technique will not dynamically balance with requisite precision because the liquid adhesive creates friction that will resist proper positioning of the balls. Also, implementation of this technique is costly.
There is a need for a dynamically balanced rotary unit with an improved balance connection method which eliminates the need for damping liquids or adhesives; achieves balancing more simply and rapidly and with more precision than means heretofore proposed; and is especially suitable for dynamically balanced disk files which have rigid shafts.